The light dimming function has been implemented in a variety of ways. Several of these implementations of dimming techniques provide a variable sinusoidal output voltage. The advantage of providing a variable sinusoidal output voltage is a lower noise level during operation, and this applies to both electrical and mechanical noise. The electrical noise in this sense refers mainly to signals back-propagated into the AC power supply that distort its almost pure sinusoidal wave-form.
One power control method that preserves the sinusoidal wave-form is to provide a high power variable resistor to control the current through a lamp and hence the intensity of the lamp. This method can be used in both direct current and alternating current electric networks. The resistor, however, dissipates large amounts of power as heat, resulting in a low overall efficiency.
Another method uses a manual or motor driven variable voltage transformer to deliver a controllable voltage to a lamp. Although the efficiency of this method is relatively high, the size, weight and cost of the equipment makes this method quite unsuitable for light dimming applications, especially when more than one variable voltage transformer has to be synchronized for parallel control of a number of lamps. Further, the cost of the motor controlling circuitry and the overall slow response when fast changing lamp intensity is required constitute additional disadvantages. The variable voltage transformer, like any other mechanical device, is subject to mechanical wear. An additional cost is incurred for an external fuse or circuit breaker to protect the internal winding from self destruction during an output overload or short-circuit.
A still further example of a method for controlling the intensity of a lamp is a multiple tap transformer. This has been used to provide output voltages in a number of increments equal to the number of taps. This method is similar to the variable voltage transformer method described above, and suffers from the same limitations.
In still another method, variations of the generic D class amplifier electronic power circuit has been used to synthesize a variable output sinusoidal voltage. U.S. Pat. No. 5,018,058 issued to Ionescu et al, describes a dual conversion high frequency switching AC controller. After the first conversion, two 60 Hz modulated unipolar variable voltage sources provide the voltages required by the output stage, designed along the class D amplifier guidelines. It is provided, however, that both unipolar voltage sources used by the output stage are not DC, but rather two half cycle waveforms, of a higher magnitude than the input voltage. Although this method could in principle be used for a light dimmer, it represents expensive overkill. Its accurate reconstruction of an ideal sinusoidal output waveform virtually independent of the input voltage waveform will impose a relative high manufacturing cost for light dimming applications, where an ideal sinusoidal output waveform is not necessary.
Aside from these sine wave maintaining systems, a relatively newer class of light dimmers uses triacs or silicon controlled rectifiers (SCRs) operating under what is generically called "variable phase angle modulation". In these methods, the triac is turned on at different phases of each half cycle of the sinusoidal wave form. This results in a large current surge through the lamp each time the turn-on event occurs. Since the internal resistance of the lamp varies with the temperature of the lamp or with the light intensity there is an increase in the magnitude of the turn-on current surge due to the fact that at each turn-on point of the triac, the filament is colder than after a number on milliseconds (or fractions of seconds for some higher power lamps) of continuous operation. This high turn-on surge current causes major mechanical and electric noise injected back into the electric network. This is a serious problem at the high current levels that would be present in a lighting system for a theater or an outdoor lighting situation such as at a ball park.
Due to the sharp electromagnetic field variations caused by triacs or SCR's and sharp thermal expansion of the filament at the turn-on point, mechanical noise is generated by the filament. This produces a train of 120 Hz (100 Hz for 50 Hz electric systems) vibrations. The intensity of such vibrations varies with the type and power level of the lamp and with the turn-on moment during each half cycle. Thus these triac systems implemented in large power consuming lighting situations result in annoying acoustic noise and severely detrimental electrical noise feedback into the power supply lines.
As a result these systems have had to go to the expense of providing equipment to attempt to reduce or eliminate both types of noise. For example, a large output inductor has been connected in series with the lamp to limit the di/dt factor by distributing the surge current at the turn-on point over a period of time of several hundreds microseconds. There is a limit to the period of time over which the surge may be smoothed without sacrificing the overall dimmer efficiency. A typical value is that the time period cannot exceed a one millisecond and this value is inversely proportional to the lamp power level. A higher power lamp will require a longer period of time for distributing the turn-on surge current than a low power lamp. The long time period associated with a system tuned for high power lamps will still cause significant amounts of mechanical and electrical noise, specially when a lower power lamp is used.
U.S. Pat. No. 4,633,161 issued to Callahan et al, describes an inductorless phase control dimmer. This patent is directed to the elimination of the filter inductor from the output stage of the dimmer. A pair of MOSFETS is slowly turned on resulting in a low di/dt factor and practically very little mechanical and electric noise. The major disadvantage of this invention is the large amount of power dissipation while both MOSFETS operate in linear mode during their turn on process. The Rds on of the MOSFETS increases with the temperature, further increasing the amount of dissipated power. A large heat sink is needed to properly dissipate the resulting heat. In the case of an output overload or short circuit, the absence of an inductor will cause a sharp output current increase, which may reach fatal levels before the internal current limiting system can react and turn off the MOSFETS.